I don't know what motivated the authors Glenn cited, but if the genus Homo
represents a single, unbranching lineage, then cladistically-inclined
systematists would see no reason to split the lineage up into different
species. A continuum like H. habilis----H erectus-----H sapiens could
easily be considered a single long-lived species undergoing change.
However, if the actual pattern turns out to be more like
H habilis---H erectus---------H sapiens
\ \ \
H rudolfensis H ergaster H neanderthalensis
then it's much more diversity to try to include under one species, and the
divergences probably merit some nomenclatorial recognition.
The basic problem is defining species-we have to have some idea of what is
a species in order to make a definition, but to know what is a species, we
need a definition!